Campylobacter spp. is part of the normal intestinal flora of a wide range of domestic and wild animals with a particular niche for the avian host. Campylobacter spp. do appear to have a limited ability to be pathogenic in domestic and wild animals animals. In cattle C. fetus subsp. jejuni and C. fetus subsp. intestinalis have been isolated from intestines and experimentally transmitted to preruminant and ruminant calves which developed clinical signs of fever, diarrhea and sporadic dysentery (Dannenberg et al. Am. J. Pathol. 34: 1099 (1958) and Thomas, Aust. Vet. J. 57: 146-148 (1981)). A syndrome of profuse watery diarrhea with fever, anorexia and depression in yearling sheep has also been reported with Campylobacter fetus as the causative agent. Campylobacter spp. has also been reported to cause clinical manifestations of dysentery, intestinal adenomatosis and hemorrhagic enteritis in pigs and horses, and mastitis in commercial dairy herds.
In humans, Campylobacter is the most commonly reported bacterial cause of endemic diarrheal illness worldwide. In the United States it is becoming the most prevalent cause of foodborne infection and affects more than 2 million people annually. In England and Wales, over 50,000 campylobacter cases are reported annually with no signs of decline of incidence. It is estimated that for every case reported to laboratory surveillance, another seven cases occur unreported. C. jejuni and C. coli are the two most commonly isolated species responsible for human Campylobacteriosis with C. jejuni now being the most frequently isolatable species.
The incubation period following ingestion of C. jejuni has been shown to be approximately 24-72 hours. The inoculum size required to induce clinical symptoms has been shown to be as few as 800 organisms. The rate of illness increases with increasing numbers of the organism ingested. Commonly reported symptoms of human Campylobacteriosis include diarrhea, fever, and abdominal cramping. Less frequently, Campylobacter, particularly C. jejuni, can cause secondary sequelae following an acute infection, including, reactive arthritis, kidney failure, Guillian-Barre, Reiter syndrome and other extra-intestinal symptoms.
The transmission of Campylobacter spp. to human populations is primarily through environmental contamination and contaminated foods, including poultry and poultry products such as eggs. Campylobacter spp. can be isolated from 30-100% of the birds in many domestic and wild avian species at any given time. In children, contact with puppies and kittens with diarrhea has been shown to be an important additional risk factor. Some additional sources of infection have resulted from drinking raw milk derived from cows having clinical mastitis caused by Campylobacter. All milk-borne outbreaks have been associated with raw or improperly pasteurized milk.
The virulence and pathogenesis of Campylobacter spp. involves both host and pathogen specific factors. Many pathogen-specific virulence determinants contribute to the pathogenesis of these bacteria. The bacterial virulence of these bacteria is the result of many different attributes, which often contribute to different steps in the complicated series of events recognized as an infection. Exposure first takes place primarily by the consumption of contaminated water, food or by direct person to person contact. Once ingested the stages of infection common to these bacteria include attachment, colonization, proliferation, tissue damage, invasion and dissemination.
The first host barrier that Campylobacter must typically overcome is the mucosal surface. A single epithelial cell layer separates the host from the lumen of the gastrointestinal tract. This barrier and a plethora of other host antimicrobial mechanisms deter commensal, opportunistic and pathogenic microorganisms from establishing infection. Adherence to mucosal surfaces is a prerequisite of this pathogen to establish infection. One of the more pronounced clinical manifestations of intestinal colonization is diarrhea. This clinical syndrome has been proposed to be produced by the synthesis and excretion of enterotoxins that cause a net secretion of fluid and electrolytes (diarrhea). Other specific virulence factors include flagella, which assist the bacterium to overcome the clearing movement of peristalsis and enable the organism to enter and cross the mucous layer covering the epithelium (Black et al., J. Infect. Dis. 157:472-479 (1988), Caldwell et al., Infect Immun. 50:941-943 (1985), Morooka et al., J. Gen. Micro. 131:1973-1980 (1985) and Newell et al. J. Hyg. Camb. 95:217-227 (1985)). Other suspected determinants of pathogenicity include chemotaxis, iron-acquisition, host cell invasion, inflammation and active secretion and epithelial disruption with leakage of serosal fluid (Black et al. J. Infect. Dis. 157: 472-479 (1988)).
Divalent metal ions such as iron, cobalt, copper, magnesium, manganese, molybdenum, nickel, selenium, and zinc are trace elements often required for the survival of bacteria infecting both animal and human hosts. These trace metal elements are used by bacteria as cofactors for enzymes that catalyze biochemical reactions for various metabolic pathways and transport systems required by the organism. The metals iron, zinc and manganese are the three most important metals required for the survival of bacteria. Zinc ions are essential for RNA and DNA polymerase activity, whereas manganese is required for mitochondrial superoxide dismutase activity. Iron is the most extensively studied of all the metal ions with direct correlations on the virulence and pathogenesis of bacteria. Iron is essential for all life and is required for enzymatic and metabolic pathways of organisms at all phylogenic levels.
The ability of Campylobacter to evade the natural defense mechanisms of the vertebrate host depends in part on its ability to obtain host iron, which in turn directly influences the host-pathogen interaction. Because of iron's essential nature, vertebrate hosts have developed elaborate mechanisms to bind iron in body fluids (e.g., transferrin in blood and lymph fluids and lactoferrin in external secretions). These high affinity iron binding proteins create an iron restricted environment within the host reducing the level of iron to approximately 10−18 molar, a concentration too low to support the growth of nearly all bacteria. These iron sequestering mechanisms of the host act as a natural defense mechanism to combat bacterial invasion. To circumvent these iron-restrictive conditions many bacterial species have evolved mechanisms for obtaining iron. The most common mechanisms include the diffusion of soluble iron through porins and specialized transport systems that mediate the uptake of iron by siderophores. This latter system is by far the most widespread or ubiquitous mechanism for iron acquisition and involves the specific chelation of ferric iron by siderophores and the synthesis of their cognate transport systems, which permits the bacteria to continue to replicate and overcome the non-specific defense mechanisms of the host. Continued replication, and thus each step in the infectious process, is ultimately dependent on the ability of the organism to obtain iron from its host.
With so many basic functions relying on the availability of iron, bacteria have evolved a complex regulatory network for acquiring iron under varying physiological conditions. Iron is a divalent cation which exists both in the ferrous (Fe2+) state and in the ferric (Fe3+) state. Under anaerobic conditions, iron is present in the soluble ferrous form (Fe2+) and can freely diffuse through outer membrane porins into the periplasm. For instance, in E. coli the FeoAB transport system present in the cytoplasmic membrane will transport the ferrous iron molecules into the cell cytoplasm. Under aerobic conditions and neutral pH, iron is primarily present in the insoluble ferric form (Fe3+) and cannot pass through the outer membrane porins by passive diffusion. Instead, molecules called siderophores are secreted by bacteria, which have a high affinity for ferric iron. The ferric-siderophore complexes are recognized by receptors in the outer membrane, collectively referred to as the TonB-dependent receptors. These receptors, once bound to loaded siderophores, are believed to interact with TonB and its associated proteins localized in the periplasm and cytoplasmic membrane. These protein-protein interactions, though poorly understood, serve to provide the energy necessary to transport the ferri-siderophore complexes across the outer membrane and through the periplasmic space. ABC transport systems present in the cytoplasmic membrane serve to transport the iron-siderophore complexes across the cytoplasmic membrane. Reductase enzymes then serve to reduce ferric iron to its ferrous form, which dissociates it from the siderophore and releases iron into the cell.
Several species of pathogenic bacteria use additional mechanisms to obtain iron from mammalian hosts, including the direct binding of heme and hemoglobin. The receptor proteins that bind these iron-containing molecules most likely rely on the TonB complex for the energy required to transport heme across the outer membrane, similar to the iron-siderophore complexes. Specialized ABC transporters are then used to transport the heme across the cytoplasmic membrane. In addition, some bacteria secrete hemophores, small molecules that can bind heme and present it to receptors on the bacterial cell surface. Several pathogenic species also produce hemolysins, which are toxins that lyse red blood cells, releasing heme and hemoglobin for uptake by the bacteria.
The outer membrane proteins of gram-negative bacteria control the selective permeability of many essential nutrients critical to the survival of bacteria, including all pathogenic bacteria that cause disease in animals and man. This selective permeability of nutrients is controlled by a class of membrane proteins called porins. It now appears that the majority of the outer membrane proteins on the surface of gram-negative bacteria are porins, identified as the general porins (e.g., OmpF), monomeric porins (e.g., OmpA), the specific porins (e.g., the maltose-specific porin LamB) and the TonB-dependent, gated porins (e.g., the siderophore receptor FepA). The porin class of proteins generally share structural features, including the presence of beta-barrels that span the outer membrane.
Little is known regarding the iron-acquisition by Campylobacter spp. Studies indicate that C. jejuni does not synthesize siderophores (Field et al. Infect. Immun. 54: 126-132 (1986) and Pickett et al. Infect Immun. 60: 3872-3877 (1992)). This data has been confirmed by sequence analysis of C. jejuni genome in which no homologs of common siderophore synthesis genes were identified. C. jejuni is limited in the iron compounds it can use as demonstrated by various feeding assays. These assays have demonstrated that C. jejuni can use the siderophores enterochelin and ferrichrome but not aerobactin, desferal, ferritin, lactoferrin, or transferrin. Therefore, it has been suggested that other iron compounds are required to support the growth of Campylobacter spp. such as heme compounds like hemin and hemoglobin, ferric iron, and ferrous iron. The fact that Campylobacter has known transport systems for siderophores, yet is unable to synthesize them, suggests that these bacteria scavenge siderophores produced by other enteric pathogens (van Vliet et al. FEMS Microbiol. Rev. 26: 173-186 (2002)).